1. Field of the Invention
The present invention relates to a foundry shell and mold composition used to form sand cores and molds for metal casting.
2. Description of the Prior Art
Phenolic novolak resins have been used for many years as a sand binder with hexamethylene tetramine as a crosslinking-curing agent to form sand cores and molds for metal casting. This is accomplished by coating sand with a mixture of phenolic novolak resin and hexamethylene tetramine to produce a free flowing product consisting of individually coated grains of the sand.
Coating the sand has been typically accomplished by at least two different methods. In the first method, the resin can be coated onto the sand particles from a solvent solution of methanol or other suitable solvent. The solvent is then evaporated as the resin and sand are mixed at temperatures ranging from ambient to somewhat above ambient. This process is known as "warm coating", and the hexamethylene tetramine is added to the resin in the form of a powder, in a mixer before the solvent has evaporated.
In the second method, solid resin can be added to hot sand, wherein it is mixed, melted and coated on the grains of sand. An aqueous solution of hexamethylene tetramine is then added to the hot resin-sand mixture. The water evaporates and cools the sand to a point where the resin solidifies, and forms a free flowing mixture of coated sand grains. This coating process is known as "hot coating" and is the most widely accepted commercial coating process used in the United States.
The resin coated sand produced by either the warm coated process or the hot coating process is then placed on a hot pattern or in a hot core box to melt the resin and bond the sand grains together while the hexamethylene tetramine acts as a curing agent to cure the resin into a durable thermoset product.
The sand molds and cores formed by this process are often in the shape of a bonded sand shell that is the negative of the mold or core shape, hence the name "shell process" for this molding method and "shell sand" for the resin coated sand.
The shell process is widely used in the foundry industry, despite having several drawbacks which limit its application. These drawbacks stem from the use of hexamethylene tetramine as the curing agent. During the reaction of hexamethylene tetramine with the resin, nitrogen is released from the hexamethylene tetramine in the form of objectionable ammonia fumes. The nitrogen that remains in the cured resins can result in nitrogen caused casting-gas defects in some types of iron and steel castings.
Accordingly, it would be very desirable to have a shell sand binder that is substantially nitrogen free, or which has a tolerable nitrogen content. Obviously, a requirement of this type would either eliminate the use of hexamethylene tetramine as a curing agent, or restrict its use to very low levels. Typically, 12 to 18 weight % of hexamethylene tetramine is currently used based upon the total weight of the phenolic resin solids. Since hexamethylene tetramine is 40% nitrogen by weight, the hexamethylene tetramine contributes substantially to the nitrogen level of the binder.
Methods of circumventing the use of hexamethylene tetramine have been suggested. For example, paraformaldehyde has been used in place of hexamethylene tetramine to eliminate the nitrogen in the cured resin. However, the use of paraformaldehyde produces formaldehyde fumes instead of ammonia fumes when the resin is cured. Moreover, paraformaldehyde cure rates are slow, resulting in decreased productivity.
It is also conceivable to use a solid thermosetting resole resin to replace some or all of the novolak resin, and thus reduce or eliminate the amount of hexamethylene tetramine used. However, resole containing coated sands tend to cake in storage, and the resins tend to age, resulting in loss of properties with time. Retaining some novolak resin would necessitate the continued use of hexamethylene tetramine, resulting in the continued presence of nitrogen, although to a lesser extent.
An article by Albanese, J. "Shell Mixing Processes and Equipment", Transactions of the American Foundrymens' Society, pages 225-228, vol. 68 (1960), covers the use of novolak resin and hexamethylene tetramine in various types of shell coating processes. Another article by Less, F. W. "Sand Coating by the Hot Process--The Method and Materials", The British Foundryman, pages 468-470, (December 1968) deals with the development of hot methods for sand coating, including preheated sand and flaked resins, and preheated sand with water borne resins, using hexamethylene tetramine as an accelerator. Johnson, C. K., "Advances in Shell and Hotbox Processes Offer Many Advantages", Modern Casting, pages 25-27 (April 1984), details state of the art improvements in both shell and hotbox technology. For example, in shell technology the availability of special sand formulations, faster curing resins, faster buildup resins, peel resistant flakes and fast shakeout sand have benefited the industry.
British Patent Specification No. 1,179,284 seeks to avoid the disadvantages of hexamethylene tetramine, which causes porosity in castings, and discloses a resin composition for use in coating a sand, comprising a resole resin and a novolac resin. The resole is prepared by reacting a phenol and an aldehyde under alcoholic alkaline conditions with a molar excess of aldehyde with regard to the phenol. The proportion of resole in the resin composition varies from about 20 to 50% by weight of the total composition.
British Patent Specification No. 1,563,686 discloses a process for coating sand with a phenol-formaldehyde resin for use in shell molds and cores. The process involves reacting phenol and formaldehyde at an elevated temperature to produce a liquid resin having a solidification temperature above about 160.degree. F., and mixing the hot resin with hot sand to coat the sand with the resin, then cooling the coated sand to solidify the resin.
U.S. Pat. No. 3,692,733 to Johnson discloses resin coated sands which are a mixture of about 0.01 to 1.0 part by weight of silicone fluid and 1000 parts by weight of free-flowing sand particles, individually coated with about 1 to 6% by weight of a solid, non-tacky layer of a potentially thermosetting resin comprising an acid catalyzed thermoplastic phenol-formaldehyde resin and curing agent.
U.S. Pat. No. 3,838,095 to Johnson et al discloses that incorporating small amounts of urea compounds into sand coated with a potentially thermosetting phenol-formaldehyde novolac resin increases both the buildup rate and cure rate of the resin coated sand.
U.S. Pat. Nos. 4,051,301, 4,134,442 and 4,197,385 all to Laitar, disclose phenolic novolac resins which incorporate a furan resin which can be used in coating foundry sands in connection with hexamethylene tetramine for use in the shell process to produce cores and molds having improved shakeout and collapsability characteristics.
U.S. Pat. No. 4,089,837 to Luttinger et al, discloses the use of resorcinol as an accelerating agent to cure a phenol-formaldehyde resin coated sand composition.
U.S. Pat. No. 4,090,995 to Smillie, discloses a process for preparing a resin coated sand used in shell molds and cores, wherein sand is mixed with phenolformaldehyde resin and at least 3% by weight of salicylic acid, the mixing being carried out at a temperature above the melting point of the resin so that a coating of the resin is formed on the sand, followed by cooling and setting the resin on the sand to solidify the resin coating.
U.S. Pat. No. 4,113,916 to Craig, discloses the incorporation of epoxy and/or phenoxy resins into sand coated with potentially thermosetting phenol-formaldehyde novolak resins improved thermal shock resistant resin coated sands, and do not create smoke and odor problems.
U.S. Pat. No. 4,196,114 to Funabiki et al discloses a process for producing resin coated sand for use in a shell mold. The process includes preheating sand with a lubricant-containing solid resole, which is said to ameliorate a caking problem and allow the resin to be obtained in an early reaction stage with increased methylol content.
U.S. Pat. No. 4,281,090 to Craig discloses that replacing phenol-formaldehyde novolak resin with up to 50% of a catechol-formaldehyde novolak resin in sand coated with a potentially thermosetting novolak resin increases the build-up and cure rate of the resin coated sand, which can then be used to make cores and molds by the shell process.